This invention relates to electrophoretic particles (i.e., particles for use in an electrophoretic medium) and processes for the production of such electrophoretic particles. This invention also relates to electrophoretic media and displays incorporating such particles. More specifically, this invention relates to novel black or dark colored electrophoretic particles.
Particle-based electrophoretic displays, in which a plurality of charged particles move through a fluid under the influence of an electric field, have been the subject of intense research and development for a number of years. Such displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays.
The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in U.S. Pat. No. 7,170,670 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays.
Nevertheless, problems with the long-term image quality of electrophoretic displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be produced using gaseous fluids; see, for example, Kitamura, T., et al., “Electrical toner movement for electronic paper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., et al., “Toner display using insulative particles charged triboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Patent Publication Nos. 2005/0259068, 2006/0087479, 2006/0087489, 2006/0087718, 2006/0209008, 2006/0214906, 2006/0231401, 2006/0238488, 2006/0263927 and U.S. Pat. Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic media appear to be susceptible to the same types of problems due to particle settling as liquid-based electrophoretic media, when the media are used in an orientation which permits such settling, for example in a sign where the medium is disposed in a vertical plane. Indeed, particle settling appears to be a more serious problem in gas-based electrophoretic media than in liquid-based ones, since the lower viscosity of gaseous suspending fluids as compared with liquid ones allows more rapid settling of the electrophoretic particles.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various technologies used in encapsulated electrophoretic and other electro-optic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. The technologies described in the these patents and applications include:                (a) Electrophoretic particles, fluids and fluid additives; see for example U.S. Pat. Nos. 5,961,804; 6,017,584; 6,120,588; 6,120,839; 6,262,706; 6,262,833; 6,300,932; 6,323,989; 6,377,387; 6,515,649; 6,538,801; 6,580,545; 6,652,075; 6,693,620; 6,721,083; 6,727,881; 6,822,782; 6,870,661; 7,002,728; 7,038,655; 7,170,670; 7,180,649; 7,230,750; 7,230,751; 7,236,290; 7,247,379; 7,312,916; 7,375,875; 7,411,720; and U.S. Patent Applications Publication Nos. 2005/0012980; 2006/0202949; 2007/0146310; 2007/0200795; 2008/0013155; and 2008/0013156;        (b) Capsules, binders and encapsulation processes; see for example U.S. Pat. Nos. 6,922,276; and 7,411,719;        (c) Films and sub-assemblies containing electro-optic materials; see for example U.S. Pat. No. 6,982,178; and U.S. Patent Application Publication No. 2007/0109219;        (d) Backplanes, adhesive layers and other auxiliary layers and methods used in displays; see for example U.S. Pat. No. 7,116,318; and U.S. Patent Application Publication No. 2007/0035808;        (e) Color formation and color adjustment; see for example U.S. Pat. No. 7,075,502; and U.S. Patent Application Publication No. 2007/0109219;        (f) Methods for driving displays; see for example U.S. Pat. No. 7,012,600; and U.S. Patent Application Publication No. 2006/0262060; and        (g) Applications of displays; see for example U.S. Pat. No. 7,312,784; and U.S. Patent Application Publication No. 2006/0279527.        
Known electrophoretic media, both encapsulated and unencapsulated, can be divided into two main types, referred to hereinafter for convenience as “single particle” and “dual particle” respectively. A single particle medium has only a single type of electrophoretic particle suspended in a suspending medium, at least one optical characteristic of which differs from that of the particles. (In referring to a single type of particle, we do not imply that all particles of the type are absolutely identical. For example, provided that all particles of the type possess substantially the same optical characteristic and a charge of the same polarity, considerable variation in parameters such as particle size and electrophoretic mobility can be tolerated without affecting the utility of the medium.) When such a medium is placed between a pair of electrodes, at least one of which is transparent, depending upon the relative potentials of the two electrodes, the medium can display the optical characteristic of the particles (when the particles are adjacent the electrode closer to the observer, hereinafter called the “front” electrode) or the optical characteristic of the suspending medium (when the particles are adjacent the electrode remote from the observer, hereinafter called the “rear” electrode (so that the particles are hidden by the suspending medium).
A dual particle medium has two different types of particles differing in at least one optical characteristic and a suspending fluid which may be uncolored or colored, but which is typically uncolored. The two types of particles differ in electrophoretic mobility; this difference in mobility may be in polarity (this type may hereinafter be referred to as an “opposite charge dual particle” medium) and/or magnitude. When such a dual particle medium is placed between the aforementioned pair of electrodes, depending upon the relative potentials of the two electrodes, the medium can display the optical characteristic of either set of particles, although the exact manner in which this is achieved differs depending upon whether the difference in mobility is in polarity or only in magnitude. For ease of illustration, consider an electrophoretic medium in which one type of particles is black and the other type white. If the two types of particles differ in polarity (if, for example, the black particles are positively charged and the white particles negatively charged), the particles will be attracted to the two different electrodes, so that if, for example, the front electrode is negative relative to the rear electrode, the black particles will be attracted to the front electrode and the white particles to the rear electrode, so that the medium will appear black to the observer. Conversely, if the front electrode is positive relative to the rear electrode, the white particles will be attracted to the front electrode and the black particles to the rear electrode, so that the medium will appear white to the observer.
If the two types of particles have charges of the same polarity, but differ in electrophoretic mobility (this type of medium may hereinafter to referred to as a “same polarity dual particle” medium), both types of particles will be attracted to the same electrode, but one type will reach the electrode before the other, so that the type facing the observer differs depending upon the electrode to which the particles are attracted. For example suppose the previous illustration is modified so that both the black and white particles are positively charged, but the black particles have the higher electrophoretic mobility. If now the front electrode is negative relative to the rear electrode, both the black and white particles will be attracted to the front electrode, but the black particles, because of their higher mobility will reach it first, so that a layer of black particles will coat the front electrode and the medium will appear black to the observer. Conversely, if the front electrode is positive relative to the rear electrode, both the black and white particles will be attracted to the rear electrode, but the black particles, because of their higher mobility will reach it first, so that a layer of black particles will coat the rear electrode, leaving a layer of white particles remote from the rear electrode and facing the observer, so that the medium will appear white to the observer: note that this type of dual particle medium requires that the suspending fluid be sufficiently transparent to allow the layer of white particles remote from the rear electrode to be readily visible to the observer. Typically, the suspending fluid in such a display is not colored at all, but some color may be incorporated for the purpose of correcting any undesirable tint in the white particles seen therethrough.
Both single and dual particle electrophoretic displays may be capable of intermediate gray states having optical characteristics intermediate the two extreme optical states already described.
Some of the aforementioned patents and published applications disclose encapsulated electrophoretic media having three or more different types of particles within each capsule. For purposes of the present application, such multi-particle media are regarded as sub-species of dual particle media.
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
A related type of electrophoretic display is a so-called “microcell electrophoretic display”. In a microcell electrophoretic display, the charged particles and the fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging, Inc. Hereinafter, the term “microcavity electrophoretic display” may be used to cover both encapsulated and microcell electrophoretic displays.
Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, U.S. Pat. Nos. 5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346. Other types of electro-optic displays may also be capable of operating in shutter mode. Electro-optic media operating in shutter mode may be useful in multi-layer structures for full color displays; in such structures, at least one layer adjacent the viewing surface of the display operates in shutter mode to expose or conceal a second layer more distant from the viewing surface.
An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; electrophoretic deposition (See U.S. Pat. No. 7,339,715); and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
However, the electro-optical properties of encapsulated electrophoretic displays could still be improved. Typically, an electrophoretic display is designed to have black and white extreme optical states; color can then be produced by providing color filters adjacent the electrophoretic medium. Many prior art dual particle displays used titania as the white pigment and carbon black as the black pigment. However, as discussed in the aforementioned U.S. Pat. No. 7,002,728, carbon black gives rise to a number of problems in electrophoretic displays, and this patent teaches replacing carbon black with copper chromite, preferably with a polymeric shell and optionally with a silica layer between the copper chromite core and the polymeric shell.
The use of copper chromite as the black pigment in dual particle electrophoretic displays does solve many of the problems associated with carbon black pigment. However, the requirements on pigments for use in electrophoretic displays continue to increase; in particular, it has been found that achieving very low reflectance of the dark state of a display is of critical importance is securing good color saturation in color electrophoretic displays using color filter arrays, and that even the dark state provided by copper chromite pigment is less than ideal for this purpose. Accordingly, there is a need for black pigments capable of providing darker states than those provided by copper chromite, and the present invention seeks to provide such black pigments. Certain pigments provided by the present invention have the additional advantage of being free from chromium, thus eliminating hexavalent chromium which may be present as a contaminant in media containing copper chromite.